I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for transmitting and receiving broadcast information in a multi-carrier CDMA communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. CDMA has been standardized by the Telecommunications Industry Association in Interim Standards IS-95A and IS-95B, entitled “Mobile Station-Base Station Compatibility Standard for Dual Mode Spread Spectrum Systems” (referred to hereafter collectively as IS-95).
In IS-95 communications systems channels of information transmitted from a common base station are distinguished from one another by orthogonal spreading codes. Each channel is spread by a unique orthogonal spreading sequence. The channels that are transmitted by an IS-95 base station include, a pilot channel, a sync channel, at least one paging channel and dedicated traffic channels. The pilot channel is used to provide a phase reference for coherent demodulation of the other channels by mobile stations within the coverage area of the base station. The sync channel carries overhead information such as timing information, pilot PN offset information and other information that allows the reception of the other overhead channels. The paging channel notifies mobile stations of mobile terminated calls directed to the mobile station in the area. Dedicated traffic channels provide information directed to the user of a specific mobile station in the coverage area of the base station.
In IS-95, when the base station sends a Sync Channel Message, it shall uses the fixed-length message format of Table 1 below:
TABLE 1FieldLength (bits)MSG_TYPE (‘00000001’) 8P_REV 8MIN_P_REV 8SID15NID16PILOT_PN 9LC_STATE42SYS_TIME36LP_SEC 8LTM_OFF 6DAYLT 1PRAT 2CDMA_FREQ11MSG_TYPEMessage type.P_REVProtocol revision level.MIN_P_REVMinimum protocol revision level. The base station setsthis field to prevent mobile stations which cannot besupported by the base station from accessing the system.SIDSystem identification. The base station shall set this fieldto the system identification number for this system.NIDNetwork identification. This field serves as a sub-identifier of a system as defined by the owner of the SID.PILOT_PNPilot PN sequence offset index. The base station shall setthis field to the pilot PN sequence offset for this basestation, in units of 64 PN chips.LC_STATELong code state. The base station shall set this field tothe long code state at the time given by the SYS_TIMEfield of this message.SYS_TIMESystem time. The base station shall set this field to theSystem Time as of four Sync Channel super frames (320ms) after the end of the last super frame containing anypart of this Sync Channel Message, minus the pilot PNsequence offset, in units of 80 ms.LP_SECThe number of leap seconds that have occurred since thestart of System Time.LTM_OFFOffset of local time from System Time. The base stationshall set this field to the two's complement offset of localtime from System Time, in units of 30 minutes.DAYLTDaylight savings time indicator. If daylight savings timeis in effect, the base station shall set this field to ‘1’;otherwise, the base station shall set this field to ‘0’.PRATPaging Channel data rate. The base station shall set thisfield to the PRAT field value shown in Table 2corresponding to the data rate used by the PagingChannels in the system.
TABLE 2Paging Channel Data RatePRAT Field (binary)Paging Channel data rate009600 bps014800 bps10Reserved11Reserved
CDMA_FREQFrequency assignment. The base station shall set thisfield to the CDMA Channel number corresponding to theCDMA frequency assignment for the CDMA Channelcontaining a Primary Paging Channel.
In IS-95 systems, each base station transmits a pilot channel that is covered only by a short PN sequence. In IS-95 systems, the short PN sequence repeats once every 26 ms. The pilot signal transmissions from each base station are distinguished from one another by a phase offset with respect to one another. In particular, there each base station associated with a single base station controller differs in phase by at least 64 PN chips.
Under normal operation, the mobile station acquires the pilot signal first. The pilot signal carries no data and is simply the all ones sequence spread by the common short code, which also spreads all of the other channels transmitted by the base station. After acquiring the pilot channel, the mobile station receives the information described above from the sync channel. The frame and interleaver timing on the sync channel are aligned with the pilot PN sequence. The zero state of the short PN sequence marks the starting point of the sync channel frame and interleaver.
In the U.S. Personal Communication System (PCS) spectrum, the CDMA channel number N specifies the carrier frequencies of the forward and reverse link channels. In particular, channel number N corresponds to a reverse link carrier frequency of (1850+0.05N) MHz and a forward link carrier frequency of (1930+0.05N) MHz, where N ranges from 0 to 1199. The bandwidth of each CDMA channel is 1.25 MHz. Therefore, the channel numbers of adjacent CDMA channels are different by at least 25 (25□0.05 MHz=1.25 MHz). To facilitate mobile stations' initial acquisition, certain carrier frequencies are designated as the preferred frequency assignments. Referring to FIG. 1, for an IS-95B CDMA system in the A block of PCS band, the channel numbers of the preferred frequency assignments are 25, 50, 75, 100, 125, 150, 175, 200, 225, 250, and 275. Mobile stations will search the preferred frequency assignments first upon powering up.
The International Telecommunications Union recently requested the submission of proposed methods for providing high rate data and high-quality speech services over wireless communication channels. A first of these proposals was issued by the Telecommunications Industry Association, entitled “The cdma2000 ITU-R RTT Candidate Submission” (hereafter cdma2000). cdma2000 proposes increasing the throughput of the forward link signals by transmitting portions of the information on three bands, each of 1.2288 MHz. This method is described as the “multi-carrier” approach.
A multi-carrier CDMA system, that uses three adjacent 1.25 MHz RF channels with channel spacing also equal to 1.25 MHz is defined in cdma2000. Referring to FIG. 2, a multi-carrier CDMA system in the A block of the PCS band can be deployed with the center channel on channel 50, 75, 100, 125,150, 175, 200, 225, or 250. Channels 50 and 250 are usually avoided to avoid interference to adjacent bands on the reverse link. In the proposed cdma2000 description, the reverse link may be direct-spread with a chip rate of 3.6864 Mcps, which makes the emission limit requirements more difficult to meet than for a chip rate of 1.2288 Mcps.
A mobile station, upon powering up, searches for pilot signal on a preferred frequency. If no pilot is found in the current channel, it changes its channel and searches again. Once a pilot is acquired, the mobile station demodulates the Sync Channel associated with that pilot to receive timing information, pilot PN offset, and other information enabling its reception of the other overhead channels.
In a multi-carrier approach, one method for providing Sync Channel data would be to tri-partition the Sync Channel message and place one third of the message on each of the three portions of the multi-carrier signal. If the Sync Channel of a multi-carrier system is spread over three channels, the mobile station will have to know the exact channels used by the system in order to demodulate the Sync Channel reliably. Since the exact channels are not known in advance, the mobile station has to try numerous combinations to receive the Sync Channel message. Given the number of preferred channels, the time spent on such trials could be excessive and therefore degrade the mobile station's initial acquisition time. Therefore, there is a need felt in the art for a method that minimizes the mobile station's search time.